Steel for machine structural use having improved machinability

ABSTRACT

Disclosed is a steel for machine structural use which is usable instead of Pb (lead) free-machining steel, has no adverse effect on environment, and can be stably machined. This steel for machine structural use comprises by mass carbon (C): 0.10 to 0.60%, silicon (Si): 0.05 to 1.0%, manganese (Mn): 0.3 to 2.0%, sulfur (S): 0.02 to 0.25%, aluminum (Al): 0.002 to 0.030%, and calcium (Ca): 0.0005 to 0.01%, with the balance consisting of iron (Fe) and unavoidable impurities, the steel satisfying Ca/Al ratio=0.1 to 1.0 in terms of % by mass ratio.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a steel for machine structural use having improved machinability. More specifically, the present invention relates to a steel for machine structural use that is used as it has been hot rolled, or after subjecting a forged steel material to various types of cutting into a predetermined shape, the shaped steel is then subjected to quenching/tempering, carburizing, induction hardening, nitriding or the like.

2. Background Art

In recent years, due to the necessity of reducing environmental loading substances, instead of Pb (lead) free-machining steel which has mainly been used in the field of steel for automobiles and the like, non-Pb free-machining steel has become desired.

S (sulfur) free-machining steel is a promising steel as the non-Pb free-machining steel. In this S free-machining steel, the machinability is improved by adding sulfur (S) to the steel to produce manganese sulfide (MnS) in the steel and acting this MnS as a stress concentration source during cutting.

Further, in order to further improve machinability of the S free-machining steel with a sintered carbide tool, inventions involving further addition of calcium (Ca) have also been proposed (see, for example, Japanese Patent Laid-Open Nos. 140853/1982, 34538/2000, and 55735/2003).

SUMMARY OF THE INVENTION

The present inventors have now found that, in a steel for machine structural use having a predetermined composition, when the Ca/Al mass % ratio is brought to 0.1 to 1.0, the steel can be used as an alternative to Pb free-machining steel and exhibits stable machinability without adversely affecting the environment.

Accordingly, it is an object of the present invention to provide a steel for machine structural use that, despite the absence of Pb which is an environmental loading element as a free-machining component, has excellent effects, for example, has excellent machinability, can realize stable machinability without causing any variation in tool life, and can contribute to a reduction in environmental loading substance.

The steel for machine structural use which is machinable according to the first aspect of the present invention comprises by mass

-   -   carbon (C): 0.10 to 0.60%,     -   silicon (Si): 0.05 to 1.0%,     -   manganese (Mn): 0.3 to 2.0%,     -   sulfur (S): 0.02 to 0.25%,     -   aluminum (Al): 0.002 to 0.030%, and     -   calcium (Ca): 0.0005 to 0.01%,     -   the balance consisting of iron (Fe) and unavoidable impurities,         said steel having a Ca/Al ratio of 0.1 to 1.0 in terms of % by         mass ratio.

The steel for machine structural use which has excellent machinability according to the second aspect of the present invention comprises by mass

-   -   carbon (C): 0.10 to 0.60%,     -   silicon (Si): 0.05 to 1.0%,     -   manganese (Mn): 0.3 to 2.0%,     -   sulfur (S): 0.02 to 0.25%,     -   aluminum (Al): 0.002 to 0.030%,     -   chromium (Cr): 0.1 to 2.5%, and     -   calcium (Ca): 0.0005 to 0.01%,     -   the balance consisting of iron (Fe) and unavoidable impurities,         said steel having a Ca/Al ratio of 0.1 to 1.0 in terms of % by         mass ratio.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram showing tool abrasion loss as a function of Ca/Al ratio in terms of mass % ratio in steel. In the drawing, graph 1 is a graph showing the relationship between crater depth and Ca/Al ratio for a sintered carbide tool, and graph 2 is a graph showing the relationship between flank wear and Ca/Al ratio for a sintered carbide tool.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors measured tool abrasion loss in turning, with a sintered carbide tool, of steels A to F with the addition of Ca and S and, for reference, steel G containing Pb shown in Table 1. As a result, as can be seen from FIG. 1, steels substantially identical to each other in Ca content and S content are different from each other in flank wear and crater depth. It has hitherto been said that, when the content of Al in the steel is high, the cutting tool is likely to be damaged due to the presence of a large amount of Al₂O₃ which is hard. TABLE 1 (mass %, Ca, Al, O, and N in ppm) Steel C Si Mn Cr S Pb Ca Al O N Ca/Al A 0.24 0.25 0.93 1.23 0.061 — 12 360 14 170 0.03 B 0.23 0.26 0.96 1.26 0.061 — 14 310 13 128 0.05 C 0.22 0.25 0.96 1.26 0.064 — 12 160 30 142 0.08 D 0.23 0.25 0.94 1.25 0.058 —  6 50 17 154 0.12 E 0.23 0.24 0.93 1.24 0.056 — 11 60 18 164 0.18 F 0.23 0.24 0.93 1.24 0.057 — 23 80 19 159 0.29 G 0.22 0.24 0.85 1.31 0.014 0.06 — 160 21 116 —

Accordingly, steels A to F as samples in Table 1 were analyzed for the content of Al₂O₃. As a result, as shown in Table 2, it has become apparent that steel having a higher Al content does not necessarily have a higher content of Al₂O₃. Further, like steel A and steel B, there were a few cases where, despite low Al₂O₃ content, the abrasion loss of the tool increased. Thus, it was found that the Al₂O₃ content does not always govern the tool abrasion property. TABLE 2 Steel Al (mass %) Al₂O₃ (ppm) A 0.036 19 B 0.031 12  C* 0.016 28 D 0.005 9 E 0.006 12 F 0.008 12 *Steel C has higher oxygen content than the other steels.

Accordingly, using Ca/Al ratio in terms of mass % ratio in steels A to F in Table 1 as an index, in FIG. 1, the relationship between the crater depth and Ca/Al ratio in the sintered carbide tool was assembled by open circles as graph 1, and the relationship between the flank wear and Ca/Al ratio in the sintered carbide tool was assembled by closed circles as graph 2. As a result, it was found that there is a good correlation between the Ca/Al ratio and the tool abrasion loss. That is, in FIG. 1, it was found that, when the Ca/Al ratio is not less than 0.1, the flank wear governing the tool life is substantially saturated, while, also for the crater depth of the tool, the abrasion loss continuously decreases with increasing the Ca/Al ratio. In this connection, in FIG. 1, the chip of the cutting tool used was type P20 specified in an ISO standard and had a radius R of curvature of 0.4 mm. Cutting was carried out under conditions of cutting speed 250 m/min, feed rate per turn 0.25 mm, depth of cut 0.5 mm, and freedom of a cutting oil.

Further extensive and intensive studies conducted by the present inventors have revealed that, in steels having a Ca/Al ratio of not more than 0.1, the sulfide is MnS substantially free from Ca and the oxide comprises a high-melting point oxide rich in Al₂O₃, whereas, for steel having a Ca/Al ratio of not less than 0.1, the content of Ca in the sulfide is increased to modify MnS to (Mn, Ca)S and, in addition, as shown in Table 3, the steels contain an oxide (CaO-AI₂O₃ or CaO-AI₂O₃—SiO₂) having an Al₂O₃ content of not more than 90% by mass on average.

Therefore, it is concluded that the reason why the abrasion property of the sintered carbide tool is improved by increasing the Ca/Al ratio is that the modified sulfide and the oxide having an Al₂O₃ content of not more than 90% by mass on average are adhered to the surface of the tool during cutting and effectively act for covering and protecting the tool.

The term “on average” in the expression “not more than 90% by mass on average” refers to average composition (% by mass) of oxide shown in Table 3. This average composition is a value obtained by analyzing each oxide of n oxides among oxides observed in section in a rolling direction in steel for the contents (% by mass) of Al₂O₃, SiO₂, and CaO contained in each oxide, and calculating the average value of n pieces of oxide for each of these contents. That is, for steel A, the results of the analysis of 14 pieces of oxide in the steel show that the content of Al₂O₃ obtained by the analysis was higher than 93.7% for some pieces and was lower than 93.7% for other pieces, but was 93.7% on average of 14 pieces. Thus, although there is a variation in composition of oxide in the steel, it was found that the machinability has correlation with the average value of the composition. In this case, the analysis is preferably carried out by EDS (energy dispersion spectrum), and, preferably, at least 10 pieces of oxide having a size of not less than 1 μm are analyzed in light of analytical accuracy and a variation in the composition of oxide. On the other hand, it should be noted that, in the Pb free-machining steel which has hitherto been used, the presence of low-melting point Pb grains finely dispersed in the steel serves to reduce cutting resistance during turning but does not have the effect of covering and protecting the tool. TABLE 3 Average composition of oxide (mass %) Steel Ca/Al n pieces Al₂O₃ SiO₂ CaO A 0.03 14 93.7 — 6.3 B 0.05 17 96.6 — 3.4 C 0.08 20 80.5 0.6 18.9 D 0.12 25 81.0 5.5 13.5 E 0.18 25 71.4 10.2 18.4 F 0.29 21 87.9 2.7 9.3 n pieces: the number of pieces analyzed

Accordingly, the steel for machine structural use which is machinable according to the first aspect of the present invention comprises, by mass, carbon (C): 0.10 to 0.60%, silicon (Si): 0.05 to 1.0%, manganese (Mn): 0.3 to 2.0%, sulfur (S): 0.02 to 0.25%, aluminum (Al): 0.002 to 0.030%, and calcium (Ca): 0.0005 to 0.01%, the balance consisting of iron (Fe) and unavoidable impurities, said steel having a Ca/Al ratio of 0.1 to 1.0 in terms of % by mass ratio.

The steel for machine structural use which has excellent machinability according to the second aspect of the present invention comprises, by mass, carbon (C): 0.10 to 0.60%, silicon (Si): 0.05 to 1.0%, manganese (Mn): 0.3 to 2.0%, sulfur (S): 0.02 to 0.25%, aluminum (Al): 0.002 to 0.030%, chromium (Cr): 0.1 to 2.5%, and calcium (Ca): 0.0005 to 0.01%, the balance consisting of iron (Fe) and unavoidable impurities, said steel having a Ca/Al ratio of 0.1 to 1.0 in terms of % by mass ratio.

In a preferred embodiment of the present invention, each of the steels according to the first and second aspects of the present invention comprises, in addition to the above steel constituents, one or at least two members selected from the group consisting of, by mass, nickel (Ni): 0.1 to 2.5%, molybdenum (Mo): 0.05 to 1.50%, vanadium (V): 0.01 to 0.50%, titanium (Ti): 0.01 to 0.50%, niobium (Nb): 0.001 to 0.30%, and boron (B): 0.0003 to 0.005%.

In a preferred embodiment of the present invention, the steel contains an oxide, and the content of Al₂O₃ in the oxide is not more than 90% by mass on average.

Thus, the present invention can provide steel products for mechanical structural use that realize stable machining. In particular, in each of the steels for mechanical structural use, the Ca/Al ratio is limited to 0.1 to 1.0, and the content of Al₂O₃ in the oxide contained in the steel is not more than 90% by mass on average. These requirements have been provided from the viewpoint of an improvement in the effect of covering and protecting cutting tools.

As described above, the term “average” in the expression “the content of Al₂O₃ in the oxide contained in the steel is not more than 90% by mass on average” refers to an average value obtained by analyzing n pieces of oxide observed in section in a rolling direction of steel, determining the contents (% by mass) of oxides such as Al₂O₃ and calculating the average value from these content values.

The reasons for the limitation of individual constituents in the steel according to the present invention will be described. In the chemical composition of the steel, “%” is by mass.

Carbon (C) is an element that is necessary for ensuring the strength of the steel. To this end, the content of carbon should be not less than 0.10%. When the carbon content exceeds 0.60%, however, the machinability of the steel is deteriorated. For this reason, the carbon content of the steel is limited to 0.10 to 0.60%, preferably 0.10 to 0.35%.

Silicon (Si) is an element that is necessary as a deoxidizer. To this end, the content of silicon should be not less than 0.05%. When the silicon content exceeds 1.0%, however, the machinability is deteriorated due to an increase in the amount of hard oxide. Further, in this case, for carburization applications, the depth of the intergranular oxide layer on the surface of the carburized layer is increased, resulting in lowered fatigue life. For the above reason, the silicon content of the steel is limited to 0.05 to 1.0%, preferably 0.05 to 0.5%.

Manganese (Mn) is an element that is necessary for ensuring hardenability. Further, manganese is an element necessary for producing MnS. To this end, the content of manganese in the steel should be not less than 0.3%. When the manganese content exceeds 2.0%, however, the machinability is deteriorated. Further, in this case, in carburized components, due to excess manganese, the depth of abnormal carburized layer in the carburization is increased, resulting in lowered fatigue life. For the above reason, the manganese content is limited to 0.3 to 2.0%, preferably 0.4 to 1.0%.

Sulfur (S) is an element that is necessary for ensuing machinability and further is necessary for ensuring chip disposability. To this end, the content of sulfur should be not less than 0.02%. When the sulfur content exceeds 0.25%, however, strength properties such as static strength and fatigue strength are deteriorated and, in addition, hot workability is deteriorated. Therefore, the sulfur content is limited to 0.02 to 0.25%, preferably 0.02 to 0.20%.

Aluminum (Al) is an element that forms a nitride to effectively suppress grain coarsening during carburization. To this end, the content of aluminum should be not less than 0.002%. Since, however, the formation of Al₂O₃ which increases abrasion of the tool should be suppressed, the upper limit of the aluminum content is 0.030%. For the above reason, the aluminum content is limited to 0.002 to 0.030%, preferably 0.002 to 0.025%.

Calcium (Ca) is an element that is necessary for regulating the form of sulfides and further has the effect of covering and protecting the tool to improve turning workability. To this end, the content of calcium should be not less than 0.0005%. When the calcium content exceeds 0.01%, however, the productivity is deteriorated and, in addition, the production cost is disadvantageously increased. For the above reason, the calcium content is limited to 0.0005 to 0.01%, preferably 0.0005 to 0.0050%.

The regulation of the Ca/Al ratio has the effect of regulating oxide and sulfide by calcium. To this end, the Ca/Al ratio should be not less than 0.1. When the Ca/Al ratio exceeds the upper limit value 1.0, however, the effect of covering and protecting the tool is saturated. Further, in this case, the sulfide becomes so hard that the service life of a drill is shortened. For the above reason, the Ca/Al ratio is limited to 0.1 to 1.0, preferably 0.1 to 0.8, more preferably 0.2 to 0.5.

In the second aspect of the present invention, the steel according to the present invention further comprises 0.1 to 2.5%, preferably 0.6 to 1.5%, of chromium (Cr). Chromium is an element necessary for ensuring the hardenability of the matrix. To this end, the content of chromium should be not less than 0.1%. When the chromium content exceeds 2.5%, however, the machinability is deteriorated.

In a preferred embodiment of the present invention, the steel according to the present invention further comprises 0.1 to 2.5% of nickel (Ni). Nickel is an element necessary for ensuring hardenability and toughness. To this end, the nickel content should be not less than 0.1%. When the nickel content exceeds 2.5%, however, the machinability is deteriorated. Further, in this case, since nickel is an expensive element, the cost is disadvantageously increased.

In a preferred embodiment of the present invention, the steel according to the present invention further comprises 0.05 to 1.50% of molybdenum (Mo). Molybdenum is an element necessary for ensuring hardenability and toughness. To this end, the molybdenum content should be not less than 0.05%. When the molybdenum content exceeds 1.50%, however, the machinability is deteriorated and, in addition, the production cost is disadvantageously increased.

In a preferred embodiment of the present invention, the steel according to the present invention further comprises 0.01 to 0.50% of vanadium (V). Vanadium is an element necessary for ensuring hardenability and toughness. To this end, the vanadium content should be not less than 0.01%. When the vanadium content exceeds 0.50%, however, the machinability is deteriorated and, in addition, the production cost is disadvantageously increased.

In a preferred embodiment of the present invention, the steel according to the present invention further comprises 0.01 to 0.50% of titanium (Ti). Titanium is an element that is effective in forming a carbonitride to suppress grain coarsening during carburization. To this end, the content of titanium in the steel should be not less than 0.01%. When the titanium content exceeds 0.50%, however, the cost is disadvantageously increased. Further, in this case, TiS which is detrimental to hot workability and machinability cannot be suppressed.

In a preferred embodiment of the present invention, the steel according to the present invention further comprises 0.001 to 0.30% of niobium (Nb). Niobium is an element that is effective in forming a carbonitride to suppress grain coarsening during carburization. To this end, the content of niobium in the steel should be not less than 0.001%. When the niobium content exceeds 0.30%, however, NbC which is detrimental to machinability cannot be suppressed.

In a preferred embodiment of the present invention, the steel according to the present invention further comprises 0.0003 to 0.005% of boron (B). Boron is an element necessary for ensuring hardenability and strengthening of grain boundaries. To this end, the content of boron should be not less than 0.0003%. When the boron content exceeds 0.005%, however, the effect is saturated.

EXAMPLE

The best mode for carrying out the present invention will be described in Example 1 with reference to Tables 4 to 7.

Example 1

100 kg of each of steels having chemical compositions shown in Table 4 was melted in a vacuum induction furnace, and the melts were cast into ingots. The ingots were heated to 1200° C. and were forged into 65-mmφ bars and 40-mm square bars. These bars were held at 900° C. for one hr and air cooled for normalizing. Thus, samples for the following tests were prepared. TABLE 4 (mass %, Ca/Al being mass % ratio) No. Classification C Si Mn P S Ni Cr Mo Cu Al Pb Ca Ca/Al Others 1 Steel of 0.19 0.25 0.85 0.013 0.080 — 1.14 — 0.09 0.005 — 0.0006 0.12 Nb = 0.03 2 invention 0.23 0.24 0.90 0.012 0.054 0.13 1.21 — 0.13 0.006 — 0.0012 0.20 — 3 0.25 0.91 0.82 0.016 0.100 — 1.06 — 0.10 0.011 — 0.0026 0.24 — 4 Comparative 0.21 0.16 0.80 0.016 0.023 — 1.18 — 0.07 0.031* 0.06 — — — 5 steel 0.22 0.28 0.89 0.012 0.052 — 1.23 — 0.09 0.015 — 0.0004* 0.03* — 6 0.25 0.85 0.92 0.021 0.102 — 1.22 — 0.13 0.010 — 0.0120* 1.20* Nb = 0.02 7 Steel of 0.18 0.24 0.93 0.016 0.100 — 1.13 0.17 0.10 0.008 — 0.0030 0.38 — 8 invention 0.21 0.16 0.81 0.012 0.053 0.14 0.95 0.21 0.15 0.006 — 0.0015 0.25 Ti = 0.05 Nb = 0.02 9 0.24 0.30 1.89 0.013 0.071 — 1.20 0.25 0.12 0.010 — 0.0012 0.12 B = 0.0008 10 Comparative 0.19 0.23 0.91 0.024 0.015* — 0.90 0.21 0.12 0.033* 0.09 — — — 11 steel 0.20 0.20 0.88 0.020 0.051 0.15 1.14 0.17 0.14 0.025 — 0.0010 0.04* — 12 0.26 0.31 1.73 0.017 0.070 — 1.23 0.20 0.14 0.011 — 0.0150* 1.36* — 13 Steel of 0.17 0.24 0.45 0.016 0.074 1.72 0.50 0.17 0.09 0.012 — 0.0020 0.17 — 14 invention 0.25 0.30 0.62 0.020 0.063 2.08 0.13 0.20 0.17 0.008 — 0.0018 0.23 — 15 Comparative 0.15 0.23 0.50 0.015 0.012* 1.81 0.45 0.23 0.10 0.024 0.07 — — — 16 steel 0.23 0.30 0.51 0.022 0.065 2.21 0.21 0.21 0.12 0.023 — 0.0015 0.07* — 17 Steel of 0.29 0.18 0.74 0.013 0.105 — 1.06 — 0.08 0.007 — 0.0023 0.33 — 18 invention 0.35 0.19 0.80 0.015 0.053 — 0.98 — 0.09 0.010 — 0.0030 0.30 Nb = 0.03 19 Comparative 0.30 0.23 0.75 0.013 0.024 — 0.98 — 0.09 0.024 0.07 — — — 20 steel 0.35 0.30 0.82 0.023 0.052 0.14 0.91 — 0.10 0.045* — 0.0018 0.04* — 21 Steel of 0.30 0.22 0.74 0.015 0.158 — — — 0.12 0.005 — 0.0030 0.60 — 22 invention 0.33 0.24 0.72 0.019 0.058 — — — 0.11 0.010 — 0.0027 0.27 — 23 0.32 0.16 0.80 0.022 0.078 — — — 0.13 0.015 — 0.0045 0.30 V = 0.05 24 0.35 0.26 0.65 0.014 0.058 — — — 0.10 0.008 — 0.0031 0.39 Nb = 0.04 25 Comparative 0.31 0.29 0.72 0.012 0.024 — — — 0.09 0.025 0.12 — — — 26 steel 0.33 0.21 0.74 0.021 0.064 — — — 0.15 0.045* — 0.0023 0.05* — 27 0.35 0.19 0.69 0.014 0.080 — — — 0.10 0.009 — 0.0130* 1.44* — 28 0.34 0.28 0.68 0.018 0.060 — — — 0.10 0.043* — 0.0025 0.06* — *values outside the scope of claims of the present invention; P and Cu: unavoidable impurities.

The above normalized 65-mm+bars were once turned into 60 mm+bars to remove oxided scale and were then subjected to sintered carbide tool turning tests under conditions shown in Table 5. TABLE 5 Test piece 60 mmφ × 200 mm Chip ISO P20; edge R 0.4 mm Machining speed 200 m/min, 250 m/min Feed rate per turn 0.20 mm, 0.25 mm Depth of cut 0.5 mm Machining oil Not used (dry type) Evaluation method Flank wear of tool after machining for 10 min; Crater depth of tool after machining for 10 min

Further, the normalized 40-mm square bars were once subjected to milling to 35-mm square bars to remove oxided scale, and a drill service life test was then carried out under conditions shown in Table 6. TABLE 6 Test piece 35 mm square × 250 mm Drill Diameter 5 mm (AISI M2 straight drill) Machining speed 30 m/min, 40 m/min Feed rate per turn 0.15 mm, 0.20 mm, 0.25 mm, 0.30 mm Drilling direction Drilling perpendicular to forging direction Depth of drilling 15 mm Machining oil Not used (dry type) Evaluation method Number of bores formed until drilling became impossible

Table 7 shows sintered carbide tool turning properties and drillability which are results of the sintered carbide tool turning test and the drill service life test. TABLE 7 Sintered carbide tool turning properties Drillability Machining Feed Flank wear Crater Machining Feed rate Drill No. Classification Ca/Al speed (m/min) rate (mm/rev) (mm) depth (mm) speed (m/min) (mm/rev) service life 1 Steel of 0.12 250 0.25 0.09 0.06 30 0.30 245 2 invention 0.20 250 0.25 0.07 0.05 30 0.30 134 3 024 250 0.25 0.06 0.04 30 0.30 312 4 Comparative — 250 0.25 0.22 0.10 30 0.30 213 5 steel 0.03* 250 0.25 0.20 0.09 30 0.30 115 6 1.20* 250 0.25 0.08 0.05 30 0.30 240 7 Steel of 0.38 250 0.25 0.08 0.03 30 0.25 208 8 invention 0.25 250 0.25 0.09 0.05 30 0.25 120 9 0.12 250 0.25 0.10 0.06 30 0.25 156 10 Comparative — 250 0.25 0.21 0.13 30 0.25 169 11 steel 0.04* 250 0.25 0.22 0.11 30 0.25 101 12 1.36* 250 0.25 0.10 0.05 30 0.25  95 13 Steel of 0.17 200 0.20 0.13 0.06 30 0.20  90 14 invention 0.23 200 0.20 0.12 0.05 30 0.20  89 15 Comparative — 200 0.20 0.25 0.11 30 0.20  76 16 steel 0.07* 200 0.20 0.22 0.10 30 0.20  73 17 Steel of 0.33 200 0.20 0.10 0.06 30 0.15 101 18 invention 0.30 200 0.20 0.12 0.06 30 0.15  83 19 Comparative — 200 0.20 0.20 0.11 30 0.15  85 20 steel 0.04* 200 0.20 0.20 0.13 30 0.15  58 21 Steel of 0.60 200 0.20 0.15 0.06 40 0.20 521 22 invention 0.27 200 0.20 0.16 0.07 40 0.20 332 23 0.30 200 0.20 0.17 0.06 40 0.20 420 24 0.39 200 0.20 0.15 0.06 40 0.20 305 25 Comparative — 200 0.20 0.26 0.12 40 0.20 467 26 steel 0.05* 200 0.20 0.26 0.13 40 0.20 265 27 1.44* 200 0.20 0.16 0.07 40 0.20 229 28 0.06* 200 0.20 0.25 0.14 40 0.20 241 *values outside the scope of claims of the present invention; underlined values are inferior to those for steels of the present invention.

In Table 7, among comparative steels, Nos. 4, 10, 15, 19, and 25 which are steels containing Pb and free from Ca do not have the effect of covering and protecting the tool in sintered carbide tool turning and, regarding sintered carbide tool turning properties, are inferior in both flank abrasion and rake face abrasion, as compared with Nos. 1 to 3, Nos. 7 to 9, Nos. 13 and 14, Nos. 17 and 18, and Nos. 21 to 24 which are steels of the present invention as steels for comparison with the comparative steels.

Among the comparative steels, Nos. 5, 11, 16, 20, 26, and 28 which are steels having a Ca/Al ratio below the lower limit of the Ca/Al ratio range specified in the present invention are extremely poor in the effect of covering and protecting the tool in sintered carbide tool turning and, regarding sintered carbide tool turning properties, are inferior in both flank abrasion and rake face abrasion to Nos. 1 to 3, Nos. 7 to 9, Nos. 13 and 14, Nos. 17 and 18, and Nos. 21 to 24 which are steels of the present invention as steels for comparison with the comparative steels.

No. 20, No. 26 and No. 28 are comparative steels having an Al content much higher than the upper limit of the Al content range specified in the present invention. Due to a high Al₂O₃ content, they are somewhat inferior in drill service life to No. 18, No. 22 and No. 24 which are steels of the present invention having substantially the same sulfur content as the comparative steels.

Among the comparative steels, No. 6, No. 12 and No. 27 which are comparative steels having a Ca/Al ratio above the upper limit of the Ca/Al ratio range specified in the present invention each are inferior in drill service life to No. 3, No. 9 and No. 23 which are steels of the present invention having substantially the same sulfur content as the comparative steels due to excessively hard sulfide, although, for the tool abrasion loss on the flank and on the rake face in the sintered carbide tool turning, these comparative steels are excellent and comparable with the steels of the present invention. 

1. A steel for machine structural use having improved machinability, said steel comprising by mass carbon (C): 0.10 to 0.60%, silicon (Si): 0.05 to 1.0%, manganese (Mn): 0.3 to 2.0%, sulfur (S): 0.02 to 0.25%, aluminum (Al): 0.002 to 0.030%, and calcium (Ca): 0.0005 to 0.01%, the balance consisting of iron (Fe) and unavoidable impurities, said steel having a Ca/Al ratio of 0.1 to 1.0 in terms of % by mass ratio.
 2. A steel for machine structural use having improved machinability, said steel comprising by mass carbon (C): 0.10 to 0.60%, silicon (Si): 0.05 to 1.0%, manganese (Mn): 0.3 to 2.0%, sulfur (S): 0.02 to 0.25%, aluminum (Al): 0.002 to 0.030%, chromium (Cr): 0.1 to 2.5%, and calcium (Ca): 0.0005 to 0.01%, the balance consisting of iron (Fe) and unavoidable impurities, said steel having a Ca/Al ratio of 0.1 to 1.0 in terms of % by mass ratio.
 3. The steel for machine structural use according to claim 1, which further comprises one or more selected from the group consisting of, by mass, nickel (Ni): 0.1 to 2.5%, molybdenum (O): 0.05 to 1.50%, vanadium (V): 0.01 to 0.50%, titanium (Ti): 0.01 to 0.50%, niobium (Nb): 0.001 to 0.30%, and boron (B): 0.0003 to 0.005%.
 4. The steel for machine structural use according to claim 2, which further comprises one or more selected from the group consisting of, by mass, nickel (Ni): 0.1 to 2.5%, molybdenum (Mo): 0.05 to 1.50%, vanadium (V): 0.01 to 0.50%, titanium (Ti): 0.01 to 0.50%, niobium (Nb): 0.001 to 0.30%, and boron (B): 0.0003 to 0.005%.
 5. The steel for machine structural use according to claim 1, wherein said steel contains an oxide, and the content of Al₂O₃ in the oxide is not more than 90% by mass on average.
 6. The steel for machine structural use according to claim 2, wherein said steel contains an oxide, and the content of Al₂O₃ in the oxide is not more than 90% by mass on average.
 7. The steel for machine structural use according to claim 3, wherein said steel contains an oxide, and the content of Al₂O₃ in the oxide is not more than 90% by mass on average.
 8. The steel for machine structural use according to claim 4, wherein said steel contains an oxide, and the content of Al₂O₃ in the oxide is not more than 90% by mass on average. 